What Size Wool Slider do I need?

What size Wool Slider do I need?

7" Mini Tray ---> Small Wool Slider

8" Mini Tray ---> Small Wool Slider

11" Mini Tray ---> Medium Wool Slider

9" Classic Tray ---> Medium Wool Slider

12" Classic Tray ---> Medium Wool Slider

15" Classic Tray ---> Large Wool Slider